This invention relates to a radar deception jammer method and apparatus and more particularly an FM barrage jamming method and apparatus which include simultaneously frequency-modulating a voltage tunable oscillator with a large-amplitude, periodic, sawtooth waveform and a small-amplitude, random waveform to provide a radar Jamming signal.
An antijamming device that is finding increasing application in modern radar is the Dicke-Fix receiver. The Dicke-Fix receiver consists of a wideband RF/IF portion, followed by a limiter, which in turn is followed by a narrowband IF amplifier. The pre-limiter gain is such that the limiter limits on receiver self-noise alone. This receiver, in its original form, is designed to counter sweep-frequency jamming. It has also been possible to adapt the Dicke-Fix to provide a constant-false-alarm-rate (CFAR) receiver for automatic detection/processing equipment. This constant-false-alarm-rate feature is essential to prevent overloading the automatic detection/processing system with false alarms in the presence of noise jamming. The CFAR feature is also used, for much the same reason, in human/PPI (manual detection) systems.
Under certain circumstances the CFAR property of the Dicke-Fix receiver can be defeated by barrage jamming. That is, a particular form of barrage can introduce false alarms into the system over a moderate range of Dicke-Fix receiver parameters. When some of the characteristics of the victim Dicke-Fix are correctly inferred, the barrage may be designed for maximum effectiveness. Such a barrage may well be more effective than white gaussian noise because of its potential ability to saturate automatic detection/processing systems and because of its ability to introduce confusing effects into manual detection systems.
The present invention provides such a barrage jamming which is referred to as Contiguous Subcarrier Barrage (CSB). The CSB is designed to be simultaneously effective against both Dicke-Fix and conventional receivers.
in addition to overcoming the CFAR property of a Dicke-Fix receiver, the CSB jamming technique is also able to suppress the radar target-echo signal (as would white gaussian noise or a suitable fast-sweep jammer). These two effects, signal suppression and false-alarm generation, should not only be effective against an automatic detection system, but also against a manual/PPI detection system utilizing a Dicke-Fix receiver. In some cases target detection in either an automatic or a manual system may be prevented by the false receiver responses alone, even though the jammer power is insufficient to suppress the target echo completely.
When a Dicke-Fix is not employed, the signal suppression effect does not occur. However, given a PPI or other intensity-modulated display, the CSB is capable of producing enough false responses in a conventional receiver to obscure the target. It is predicted, however, that the CSB will not be capable of producing a sufficient number of false responses to Jam effectively a conventional receiver with an A-scope display. Thus, the CSB is not considered suitable for jamming tracking radars.
Some clarification in the usage of the terms "false response" and "false alarm" is in order. A false response can be generated each time a subcarrier sweeps through the Dicke-Fix post-limiter passband. A single false response is seldom mistaken for a target; i.e., a false response does not, in general, constitute a false alarm. As used here, a false alarm consists of a number of false responses which, following integration or some other form of processing, appear to be a legitimate target. A false response is defined on the following basis. Assume that the only signal at the receiver input is noise (self-noise and/or jamming). This noise is detected and appears in the video portion of the receiver system, in which a threshold-voltage level has been previously set. If, during a time interval equal to the radar's pulse width, the video noise voltage exceeds the threshold value, a false response is said to have occurred. Normally, a number of false responses are required to meet some detection criterion before a false alarm (false target) is declared to be present. In automatic detection systems, the detection criteria are known and one may therefore estimate the false-alarm probabilities, knowing the false-response probabilities.
Using maximum false-response probability as a criterion, the optimum bandwidth for the secondary random (noise) modulation for the CSB has been determined. This bandwidth is a relatively noncritical function of the signal bandwidth of the victim radar.